As watches, there are two types of mechanical watches which operate by the use of power of a mainspring and electronic watches which operate by electric power of battery loaded therein. Each of the electronic watches and the mechanical watches has a train wheel portion to move hour hand, minute hand and second hand, such as wheels and bridges, and a slide portion such as levers. For the train wheel portion and the slide portion, parts made of metals or plastics are used taking processability and strength into account.
For the operation of the hands of watches, a magnetized rotor rotates 180° for one second and this rotation is transmitted in the following manner. That is to say, the rotation of the rotor is transmitted to a fifth wheel and pinion, a fourth wheel and pinion, a third wheel and pinion, a second wheel and pinion, a minute wheel, and an hour wheel in this order, and the fourth wheel and pinion moves the second hand, the second wheel and pinion moves the minute hand, and the hour wheel moves the hour hand, whereby each hand is operated.
Watches usually have a time-adjusting function. When a crown is pulled in order to adjust time, a clutch wheel is geared into the minute wheel. When the crown is revolved in this state, the clutch wheel is rotated to thereby rotate the minute wheel. By the rotation of the minute wheel, the hour wheel is rotated, whereby the hour hand can be moved. By the rotation of the minute wheel, the second wheel and pinion is also rotated, whereby the minute hand can be moved.
The minute wheel, however, is interlocked with the rotor through the second wheel and pinion, the third wheel and pinion, the fourth wheel and pinion, and the fifth wheel and pinion, so that if the crown is revolved, even the rotor is rotated. Then, in order to prevent rotation of the rotor caused by adjusting time, watches are equipped with a braking mechanism and a sliding mechanism to rotate only wheels necessary to adjust time. The sliding mechanism is usually set on the second wheel and pinion.
The sliding mechanism has an appropriate torque (referred to as “slip torque”), and when a force higher than a certain torque is applied, the sliding mechanism is activated, and thereby, rotation is not transmitted between the second wheel and pinion, and the third wheel and pinion. More specifically, in the usual motion of hands, the rotation is transmitted from the third wheel and pinion to the second wheel and pinion, but when the crown is revolved, a force of a certain torque is applied to actuate the sliding mechanism, whereby rotation is not transmitted from the second wheel and pinion to the third wheel and pinion.
However, if the time-adjusting operation is repeatedly carried out, the sliding mechanism suffers frictional wear and is deteriorated to thereby lower the slip torque. Consequently, it becomes difficult to stop the hand at the desired position in the time-adjusting operation, or also in the usual motion, the sliding mechanism sometimes is activated to thereby stop the motion of the minute hand.
Therefore, a lithium soap grease containing as a base oil an ester type synthetic oil or a mineral oil is conventionally poured into the sliding mechanism to prevent deterioration of the sliding mechanism caused by frictional wear and thereby inhibit lowering of torque. However, if a synthetic oil having a large total acid number and exhibiting metal corrosiveness (e.g., Mabis 9415) is used for a metal part of a precision instrument such as a watch, the metal part is occasionally tarnished or dissolved. Further, if a grease (e.g., CH-1 available from Citizen Watch Co., Ltd.) having poorer storage stability than a high-purity synthetic base oil (e.g., International Publication No. WO01/59043) is used, there is brought about a problem that the sliding mechanism is immediately deteriorated. On this account, development of grease having a small total acid number and exhibiting excellent storage stability has been desired.
Furthermore, grease having been poured into the sliding mechanism sometimes mingles with a lubricating oil that has been applied in order to slide the second wheel and pinion. As a result, deterioration of the slide portion or change of properties of the lubricating oil sometimes occurs. For example, if the aforesaid Mabis 9415 is mixed with the lubricating oil, metal corrosiveness of the lubricating oil is increased to sometimes deteriorate the slide portion. If the CH-1 available from Citizen Watch Co., Ltd. is mixed with the lubricating oil, change of properties of the lubricating oil takes place and the properties inherent in the lubricating oil cannot be obtained in some cases.
Then, as a sliding mechanism having an appropriate torque, a second wheel and pinion manufactured in combination with a resin has been proposed (Japanese Patent Publication No. 16705/1996, Japanese Patent Laid-Open Publication No. 123783/1994, Japanese Patent Laid-Open Publication No. 196747/1993). This second wheel and pinion is employable without oil-feeding and is prevented from mixing of a lubricating oil, but it is difficult to easily manufacture the second wheel and pinion because of its complicated structure. Moreover, there is another problem that the sliding mechanism has poor wear resistance because it is made of a resin.
Other various grease compositions have been heretofore proposed (e.g., Japanese Patent Laid-Open Publication No. 31706/1978, Japanese Patent Laid-Open Publication No. 35963/1999, Japanese Patent Laid-Open Publication No. 336760/1999, Japanese Patent Laid-Open Publication No. 336761/1999, Japanese Patent Laid-Open Publication No. 172656/2001, Japanese Patent Laid-open Publication No. 308125/2002), but these grease compositions are intended for large-sized machines, and their consistency is large. Therefore, even if these grease compositions are used for sliding mechanism of watches, it is difficult that the sliding mechanism has a suitable torque.
It is an object of the present invention to provide a grease composition for a precision instrument which has no metal corrosiveness, hardly suffers change of properties and can maintain an appropriate slip torque in a precision instrument such as a watch. It is another object of the invention to provide a watch which exhibits stable operating performance by the use of the grease composition for its sliding mechanism.